The extinction of New Zealand's giant, flightless moa birds may have been hastened by the long time they took to reach maturity, experts believe.

UK and New Zealand scientists studied growth rings (similar to tree rings) in leg bones from the giant birds.

They found that moa took about 10 years to reach full size and then several more to reach sexual maturity.

This left them vulnerable to human hunters who got to New Zealand 700 years ago, the team writes in Nature.

The hunters may simply have wiped out the birds by picking them off before they had a chance to become parents.

Sam Turvey, of the Zoological Society of London, and colleagues argue that their slow growth was the result of a particular life history strategy.

It may have enabled them to invest their reproductive effort toward the production of a few, large young.

This was possible because moa lived on an island with no natural enemies apart from giant eagles. But this all changed with the arrival of Polynesian colonists in about AD 1300.

"Moa are closely related to many living birds, including ostrich, emu and cassowary, which suggests that all birds may have this inherent ability for extending their development in this way," said Dr Turvey.

"However, if birds evolve in the presence of predators, the emphasis is instead on more rapid reproduction."

Big bird

By contrast with moa, living birds tend to reach adult size within 12 months.

The largest moa, called Dinornis, could reach 2m (6.5ft) tall and weigh up to 240kg (530lbs).

Evidence suggests moa almost certainly died out within 100 years of Polynesian settlers arriving in New Zealand.

A study published last November postulates that there was a crash in the moa population that occurred before the arrival of the first settlers.

According to this work, moa numbers may have been between three million and 12 million birds 1,000 years ago but tumbled to just 159,000 before the Polynesians arrived, possibly because of avian diseases brought by migrating birds or local effects of volcanic eruptions.